NTA Monitor discovered a denial of service vulnerability in the Cisco VPN 3000 series concentrator products while performing a VPN security test for a customer in July 2005. The vulnerability affects Phase-1 of the IKE protocol. Both Main Mode and Aggressive Mode over both UDP and TCP transports are affected. The vulnerability allows an attacker to exhaust the IKE resources on a VPN concentrator by sending a high rate of IKE requests, which will prevent valid clients from connected or re-keying. The attack does not require a high bandwidth, so one attacker could potentially target many concentrators. This mechanism behind this vulnerability is similar to the well-known TCP SYN flood vulnerability.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/48634/ciscoVPN.txt
Source: https://packetstormsecurity.com/files/48634/ciscoVPN.txt.html